It is known to electro-optically scan the length of fabric in at least one zone of strip form by sensing means, which for recording the state of the fabric emit electrical identifier sensing signals at a number of sensing points lying within the strip-form zone, which for the purpose of identifying faults are analyzed in such a way that distinction is made between different forms and/or sizes of different types of fault. Separate output signals are produced for types of fault detected in this way and for energizing display and/or control or switching devices. A relative movement of predetermined speed in a direction transverse to the strip-form zone is maintained between the length of fabric and the sensing means during scanning. Zones of the length of fabric to be scanned are preferably scanned a number of times.
In high-speed circular knitting machines for example there is a need for the tubular fabric being taken off continuously during operation to be monitored for the occurrence of faults. These faults may be "spot" faults, i.e. "holes" as caused for example by a thread break and running transversely to the lengthway direction of the tubular fabric, mainly over several wales of stitches. There can also be "long" faults, however, i.e. faults which extend longitudinally along the tubular fabric over several courses of stitches and typically occur as "runs". With spot faults occurring only spasmodically such as when knitting yarns of inferior quality, or with "long" faults of relatively short length, it is often not worthwhile stopping the machine. It is desirable, however, to record the number of these faults occurring for instance per unit time or in the fabric roll. However, if runs suddenly start to occur in the tubular fabric which extend over a considerable length, then this a sign of a needle breakage having occurred and that the machine needs to be stopped as quickly as possible.
A variety of textile monitoring devices with electro-optical sensing heads and associated analysis units are known for the continuous monitoring of a length of textile fabric, especially a tubular knitted fabric produced on a circular knitting or warp-knitting machine. These sensing heads operate with light sources and light detectors in the form of photocells, frequently arranged in rows and responding to the change in the reflectance status of the fabric occurring consequent upon a fabric fault and emitting corresponding signals. These signals are analysed by the various known methods according to a variety of criteria. When the result of the analysis exceeds a prescribed tolerance value the machine is switched off.
Many of the previously known sensing devices have the disadvantage that although they respond generally to spot faults and long faults, they are unable to undertake any reliable fault discrimination according to the type of fault involved. In order to prevent unnecessary interruptions in the knitting process, however, it is advisable to prescribe a permissible fault rate and only emit a stop signal to the machine on reaching this fault rate.
Greater importance is thereby attached to the detection of long faults in the form of runs, dropped stitches, pulled stitches, i.e. faults which cause a fault to be recorded repeatedly at every revolution of the machine. As already mentioned, long faults of considerable length are invariably due to a needle defect at the point of stitch formation, which it is essential to correct quickly to prevent faulty lengths of knitted fabric.
On the other hand, however, abortive stoppages for short-term fault phenomena in the knitted fabric (e.g. thick or thin places in the yarn, trash in cotton etc.) must be positively prevented, as they adversely affect the productivity of the knitting machine and largely represent an interference factor. Examples of such known devices and methods for monitoring a length of textile fabric with electro-optical sensing heads may be found in a number of publications:
From DE-OS 1 938 677 a run detector for circular knitting and warp-knitting machines is known which is arranged within the tubular knitted fabric and which has a light beam source together with at least one photodiode as the beam detector, which are arranged at uniform distance from the tubular fabric and of which the photodiode is located in relation to the beam source in such a way that it is incapable of detecting any direct illumination from the beam source. Suggested sources of illumination here are standard light sources or luminescence diodes operating in the infrared zone. Details of how the control circuit which analyses the signals emitted by the light detectors is designed are not disclosed.
In a method known from DE-PS 3 133 428 for detecting and analysing faults in textiles, especially in knitted fabrics, a number of emitter heads are used, in which the signal analysis circuit comprises a microcomputer. A textile fabric monitoring device similarly operating with a microprocessor in accordance with DE-PS 3 536 991 permits automatic adjustment of its sensitivity to the level of reflectance of the length of textile to be monitored, in order thereby to improve the confidence level of the fault record. A number of infrared light diodes arranged in a row are used as pulsed light emitters.
In a method known from DE-OS 4 001 650 for monitoring faults in a length of textile fabric, especially on circular knitting machines for detecting runs, bars, needle stripes and holes, precautionary measures are provided so that regardless of the speed of the machine, the fault monitoring device is calibrated automatically and the machine switch-off is actuated only in the case of a fault being detected repeatedly at the same point. The number of repeated occurrences of a fault needed for switch-off can be adjusted as required. However, to prevent unnecessary stoppages only faults of the nature of a run are intentionally considered. The electro-optical sensor comprises infrared diodes and phototransistors arranged alternately in a row.
Finally a device is known from DE-OS 2 644 502 for detecting holes in a piece or length of fabric.
This system is intended to discriminate between holes extending transversely to the direction of travel of the fabric, i.e. holes of the nature of a run and relatively small holes. For this purpose electro-optical sensing means are used which incorporate a slot arranged transversely to the direction of travel of the fabric and on which a number of photocells are arranged, the output signals of which are compared with each other. An analysis circuit compares the output signal of each photocell with the mean of the output signals of the remaining photocells and identifies on a symmetrical or an asymmetrical signal distribution the presence of a spot fault or long fault. The identification of long faults (runs) and their differentiation from holes is not possible by this means with a sufficient confidence level, because it is assumed that the long faults run exactly parallel to the photocells arranged in a perpendicular row. Such a requirement is practically impossible to satisfy on a circular knitting machine, however, because the tubular fabric with its initial cylindrical shape must of course be laid flat for takeup on the fabric roll. If a run lying only slightly obliquely to the row of photocells passes, then consecutive staggered individual signals are produced in the latter, the result of which is that the run is erroneously interpreted as holes occurring in quick succession.